Donald Trump is threatening an online internet campaigner with a $25m lawsuit over a campaign to encourage Macy's to drop Trump's products from its shelves and his face from its advertising.

Angelo Carusone, who set up the 'Dump Trump' campaign last year, received a letter from Trump's attorney warning of "a major, multimillion dollar lawsuit" unless Carusone stops attempting to "tortiously interfere with Mr Trump's business and contractual relationships".

On Tuesday, Trump's attorney told the Guardian that Carusone was "a loser", but it would appear that the business tycoon's latest threat of legal action – he is already in the process of suing comedian Bill Maher over claims of orangutan heritage – has already backfired, with Carusone promising to take his campaign to "the next level" over the coming weeks and months.

"I am litigation counsel to Donald J Trump and hereby demand that you immediately cease your malicious efforts to tortiously interfere with Mr Trump's business and contractual relationships," Garten wrote. "Failing which, we will have no choice but to commence a major, multi-million dollar lawsuit against you, your organisation and co-conspirators."

The letter, dated 27 December, said Carusone's conduct had "far exceed[ed] anything protected by the constitution" in seeking to disrupt Trump's business relationship with Macy's "through mob-like bullying and coercion".

"You are hereby forewarned that in the event that you continue your unlawful actions, including, without limitation, your interference with contract, Mr Trump will immediately sue you for compensatory, consequential and punitive damages, and seek not less than $25,000,000.00 in damages," Garten's letter concluded.

The petition, which is still live and was continuing to attract signatures on Tuesday, says Macy's should drop Trump due to his "especially unpleasant, nasty and despicable behaviour". Carusone, who lives in Washington DC, delivered the petition to Macy's in New York last November.

The 30-year-old told the Guardian that he had recognised the letter as "something which needs to be dealt with seriously", and had spent January arranging legal representation before publishing the letter to the web on Tuesday. He said he had considered ending the 'Dump Trump' campaign in the face of the potential legal action.

"These things can get messy, and I'm aware of that, and Donald Trump is volatile, and extremely wealthy, so that's a potent combination right there, and he seemed very frustrated," Carusone said.

But he said he had decided instead to continue with the campaign, as "for me the worst thing is to succumb to bullying and intimidation". He said his lawyers had contacted Trump's attorney asking for evidence of where Carusone had been defamatory and had not heard back. His lawyers were working pro bono, Carusone said, and believed he was safe from the potential $25m suit.

Carusone said the campaign could resume the deployment of anti-Trump mobile billboards at Macy's sites. The campaign had vehicles circle Macy's stores in Ohio and New York last year, and Carusone said planning was in place around "expanding those efforts to other cities and localities" and potentially protesting Macy's events.

"I think that Macy's has a stable of relationships and they do lots of events with some of their partners. When you're having one of these singers or musicians down to Macy's to the next big event that they do, having some big event there in the form of an advertisement or individuals is part of the process too."

That news is unlikely to please the outspoken star of the US Apprentice, who has a history of taking legal action against those taking the Trump name in vain. The letter to Carusone hints at Trump's litigious past, urging him to "look no further than former Miss Pennsylvania Sheena Monnin, who just last week found herself on the wrong side of a $5m judgment in favour of Mr Trump after falsely stating in the press that the Trump-owned Miss USA pageant was both "fixed" and "trashy".

Garten told the Guardian that Carusone "is desperate for attention" and said it was a "dead story", although he said it was still possible that Trump could sue for $25m.

"It's a dead story. I wrote the guy a letter two months ago, it's dated December 27th, it's had no impact on sales, Mr Trump's line is doing phenomenally well at Macy's, and the guy is just trying to revive a dead story."

Asked why the legal threat was made if the campaign had made no impact, Garten said: "Sales are doing great, but it doesn't mean we're gonna sit back and allow a guy like this, who is a loser, to interfere with agreements we have with Macy's and others."

Garten said: "I don't know. We'll see how this thing plays out," when the Guardian asked if Trump would be pursuing legal action. He denied that Trump might be being oversensitive in his reaction to Carusone.

"Not at all. He's worked long and hard to develop his brand, and when some loser like Carusone attempts to interfere with that in order to capitalise on media attention, then you know, it's something you have to protect."